Automotive fuel supply systems usually incorporate thermally responsive choke controls which regulate choke valve movement in a carburetor to improve engine starting at various ambient temperatures while also achieving improved fuel efficiency and improved pollution emission control. Such choke controls typically include a coil spring of thermostatic bimetal which is connected directly to an unbalance-mounted, air-movable choke valve. The thermostatic spring is selected so that when the engine is started and when engine vacuum tends to pull air into the carburetor to move the air-movable choke valve toward an open position, the spring resiliently biases the choke valve toward its closed position, thereby tending to provide a relatively richer fuel mixture to the engine. On a cold day, when a very rich fuel mixture is desired to permit smooth engine startup, the thermostatic spring provides a substantial force biasing the choke valve toward its closed position. However, on a warmer day, the spring responds to the higher ambient temperature and provides a relatively smaller choke valve biasing force as the engine is first started. In either event, the thermostatic spring is arranged to increase in temperature as the engine warms up to provide a progressively decreasing choke valve biasing force, thereby to permit a progressively leaner fuel mixture to be drawn into the engine to improve fuel efficiency and to reduce emission of unburned hydrocarbons and the like in the engine exhaust as warm up is achieved.
Many conventional choke controls incorporate electrically operable heaters which are energized to transfer heat to the thermostatic spring when engine operation is initiated. Such controls are adapted to provide strong, initial choke valve closing forces but permit the choke valve to be moved relatively rapidly to fully open position as the engine warms up. Other controls incorporate thermostatic switches which initiate operation of such heaters only when ambient temperature is above a selected level or only after a degree of engine warm-up has occurred. Such controls tend to provide a slow initial decrease in choke valve biasing force but then provide more rapid decrease in the force after the heater is energized to reduce pollution emission at the end of the warm-up cycle. Other controls use plural electrical heaters, one of which is operable by a thermostatic switch, to provide a slow but definite initial rate of change of choke valve biasing force on a cold day and to provide more rapid change in biasing force on a warm day or as engine warm-up nears completion. Other controls use hot air transfer means and the like to transfer heat to the thermostatic spring from the engine or use heat-conducting means to provide different heat transfer paths between plural heaters and the thermally responsive spring, thereby to provide the choke controls with particular performance characteristics as may be desired. Frequently however, considerable difficulty is experienced in trying to match the performance characteristics of a thermally responsive choke control to the requirements of a particular carburetor or engine under the different ambient temperature conditions likely to be encountered. Significant compromises often have to be made and, in any event, a considerable amount of design engineering effort is required to develop a choke control to meet the needs of each different carburetor or engine presently in use.
It is an object of this invention to provide a novel and improved automotive fuel supply system which achieves improved engine starting at various ambient temperatures while also achieving improved fuel efficiency and pollution emission control; to provide such a system which is adapted to meet the performance requirements of various different carburetors and engines; to provide such a system which is adapted to be easily modified to meet different performance requirements for different carburetors and engines; and to provide thermally responsive choke controls for use in such fuel supply systems.